In the case of the mills cited, their throughput is controlled by means of adjustments to various manipulated and reference variables, such as for example the rotation rate or rotational speed of the drum, the feeding of the coarse grained ore, a water feed and/or the speed of discharge of the milled material present at the outlet. An important quality attribute here is the distribution of grain sizes in the milled material. This affects the yield from other components downstream from the mill, such as for example a flotation facility. The objective is to achieve as high a throughput as possible with high product quality and low costs. The energy and/or material requirements make a major contribution to the last of these.
Today's mills are adjusted manually by the operating staff on the basis of empirical values from their experience. Many drum mills, in particular of older designs, can only be operated at a single rotational speed or rotation rate, which is laid down back at the development stage of the mill. In this case, the rotation rate cannot be controlled. In contrast, newer mills, such as for example mills with a direct drive or gearless drive, as applicable, have an ability for their rotation rate to be adjusted to any required set value over a wide range.
There are known control units for mills which select an optimal rotation rate for the mill and hold this constant during the operation of the mill. Here, the rotation rate can be adjusted beforehand for the different types of ore or other operating conditions which are relevant for the mill. A rotation rate regulator for a tube mill is known from DE 10 2006 038 014 B3.
A known way of improving the discharge characteristics is to output the material not in the middle but at the wall of the mill. Mills which do this are referred to as screening drum mills. However, screening drum mills are not suitable for processing ores, because suitably robust sieves can only be constructed with difficulty. A known alternative is to construct the lifter plates differently. In this case, the typical straight-line radial lifter plates are replaced by curved or even more complex 2-chamber structures, as known from U.S. Pat. No. 7,566,017 B2.
An adaptive model predictive regulation for a tube mill is known from DE 10 2006 019 417 A1.